As the use of portable electronic devices, such as laptop computers, mobile phones, video cameras, digital cameras, etc., has been increased, the demand of secondary batteries having high energy density that are used as power sources of the portable electronic devices has been increased. In addition, the demand of secondary batteries for electric vehicles has also been increased: One of the secondary batteries is a lithium-ion battery including anodes made of a carbonaceous material, cathodes made of lithium metal oxide, separators made of polyolefin, and an electrolyte.
However, the lithium-ion secondary battery has a high operating potential. As a result, high energy may instantaneously flow, and therefore, chemical activity is greatly increased due to overcharge or short circuits. Consequently, a large amount of gas is generated due to abrupt response with the electrolyte. As a result, the inner pressure or the inner temperature of the battery is abruptly increased, and therefore, when the battery explodes, apparatuses adjacent to the battery may be damaged, or a human body may be injured.
For this reason, various safety devices to prevent the explosion of the lithium-ion secondary battery are mounted in the lithium-ion secondary battery. For example, U.S. Pat. No. 5,738,952 discloses a structure in which a safety plate and a cathode lead tap are connected with each other by welding, and, when the inner pressure of a battery reaches a predetermined critical pressure level, the safety plate is turned over, and therefore, the welding region between the safety plate and the cathode lead tap is broken to intercept current, thereby preventing the increase of the pressure and the temperature of the battery. Consequently, when the inner pressure of the battery reaches the predetermined pressure level, the current flow route is intercepted, and therefore, the increase of the pressure is prevented. In the conventional art, the safety plate is mounted at the outside of the battery. Consequently, the current flow at the outside of the battery is intercepted when the abnormal response of the battery occurs. However, energy has already been accumulated in the battery, and therefore, the battery may explode when overcharge is continuously performed or when the inner temperature of the battery increases.
In addition, there have been proposed a technology for melting a heat fuse, when a battery cell swells, to accomplish a short circuit of the battery cell (Japanese Unexamined Patent Publication No. 2003-231319) and a technology for actuating an intercepting switch, when a battery cell swells, to intercept current. According to these technologies, however, energy has also been accumulated in the battery cell. Consequently, the battery cell may explode when overcharge is continuously performed or when the inner temperature of the battery cell increases.
As a method to solve the above-mentioned problems, there has been proposed a technology for discharging a battery cell based on a signal from a pressure detection means (a pressure-sensitive resistor) for detecting the swelling of the battery cell (Japanese Unexamined Patent Publication No. 2001-243991). According to this method, when the battery cell swells due to overcharge, the pressure of the battery cell is detected to drive a FET. That is to say, the FET is driven by the operation of a circuit, and therefore, such a function must be additionally included in the circuit. When the circuit is abnormally operated, however, the safety device is not operated. Consequently, when the battery is exposed to high temperature or the battery is overdischarged due to the abnormality of the circuit, the normal operation of the safety device through the circuit is not guaranteed.
As another method of consuming electrical energy accumulated in a battery cell through a discharge resistor, there has been proposed a structure in which a conducting member is spaced a predetermined distance from a battery case, which is also used as an electrode terminal (+), the conducting member being connected to another electrode terminal (−), and, when the battery case swells, the conducting member and the battery case are brought into contact with each other, whereby a short circuit occurs, and therefore, the battery cell is discharged (Japanese Unexamined Patent Publication No. 2004-319463). However, this method has disadvantages in that the battery case is used as the electrode terminal, i.e., the construction of the battery is restricted, and reliable operational characteristics are not provided.